Thanks for your advice everyone!
I guess I'll have to wait and see if it works when I get home. I thought I noticed a faintly burnt smell coming from the device after I plugged it in, so it may be toast. And to those who asked, the nameplate reads 120/240 volts and 50/60 hertz. There is no switch between them. I asked at a local travel store before leaving home and the the guy said dual voltage heating devices should be no problem.
What I am still confused about is why it would work perfectly, multiple times in Milan but then blow a fuse in Florence.
Kim: I would keep your receipt in case something happens to your dual voltage device. And be prepared for the lights to go out. Mine is a couple years old so I probably can't get money back if it's fried. It was purchased from a beauty supply store through a hairdresser friend and cost upwards of $200 - intended for professional salon use. I bought another (European) one here yesterday for 65 euros. That was after shopping around, other places were asking around 120 euros. A good straightening iron is not cheap.
Which brings me to my next question: if I buy a voltage converter when I get home, can I use the European straightening iron in North America?
This might seem trivial to some, but the other women out there who regularly use straightening irons will understand :)
Especially when touring fashionable Italian cities! I also have some formal, work-related events to attend while in Europe where it's important to look pulled together.